Recently, semiconductor laser diodes have come to be widely used in a variety of types of electrical equipment, for example, laser printers, optical disk apparatuses, fiber-optic communication apparatuses, and mobile phones, because of their compact size, low cost, and ease of use.
However, the current/quantity of light characteristic of the semiconductor laser diode is dependent on temperature. Accordingly, it is necessary to control quantity of light to obtain a predetermined quantity of light reliably. This quantity of light control is called Automatic Power Control (APC). In the APC process, before the semiconductor laser diode is actually driven, the laser diode is driven in advance, the quantity of light from the laser diode is received by a photo diode (PD), and the detection current values of corresponding quantities of light are stored in a storage device. Then, the semiconductor laser diode is controlled using the current values saved in the storage device so as to obtain a desired quantity of light reliably.
FIG. 1 is a schematic diagram illustrating a related art image forming apparatus 200X. As illustrated in FIG. 1, the image forming apparatus 200X may be a copier, a facsimile machine, a printer, a multifunction printer using electrophotographic process used in a copier, a facsimile machine, a printer, a multifunction printer having at least one of copying, printing, scanning, plotter, and facsimile functions, or the like.
In FIG. 1, the image forming apparatus 200X includes a semiconductor laser unit (edge emitting laser unit) 1, a polygon mirror 2, a scanning lens 3, a photoreceptor 4, a beam sensor 5, an image control unit 6, and a laser driver 7.
In an image forming process, a semiconductor laser (laser diode) LD (edge-emitting laser unit) functioning as a lighting source in the semiconductor laser unit 1 emits a laser beam, and the laser beam is scanned (deflected) by the polygon mirror 2 that rotates at a predetermined velocity. Then, the lased beam forms a lighting spot on the photoreceptor 4 (scanned medium) via the scanning lens 3 (f θ lens).
The deflected laser beam scans and exposes in a main scanning direction orthogonal to a sub-scanning direction in which the photoreceptor 4 rotates, and records image signals with respect to each line thereof.
The beam sensor 5 is disposed at a position at which the laser beam is radiated, that is, a position close to the one end of the photoreceptor 4, to generate a main scanning synchronized signal. The image control unit 6 generates image data Di and automatic power control (APC) timing signal Sapcsh synchronized with the main scanning synchronized signal. The semiconductor laser driver 7 controls power of lighting of laser diode LD based on the APC timing signal Sapcsh, and emits the semiconductor laser LD, synchronized with the image data D.
While the semiconductor driver 7 controls the emission time of the semiconductor laser LD based on the APC timing signal Sapcsh generated in the image control unit 6, the laser LD repeatedly scans on the photoreceptor 4 in the main scanning direction at a predetermined cycle in accordance with the rotation velocity and the recording density, thus forming a latent image on a surface of the photoreceptor 4.
In the above-described beam scanning-type image forming apparatus, the laser beam is deflected by the polygon mirror 2 at equal angular velocities, and in order to keep the scanning speed on the scanned medium (photoreceptor) constant, fθ lenses and fθ mirrors are used.
Although the scanning speed of the laser beam deflected by the fθ lens or the fθ mirror on the scanned medium is substantially constant, light-emission intensity of the laser beam on the scanned medium may change based on image height of fθ lens. “Image height” is a distance from the center of the photoreceptor in a main scanning direction of the photoreceptor. Light-use efficiency, for example, transmissivity and reflectance of optical elements (glass, lens, and mirror) through which the laser beam emitted from the semiconductor laser LD onto the scanning medium (photoconductor), differs depending on an incident angle of the laser beam, and the thickness of the fθ lenses are different, thus resulting in the fluctuation of beam intensity (light emission amounts) based on the image height. The fluctuation of the beam intensity based on the image height is called “shading characteristic”.
Fluctuation in the beam intensity due to the shading characteristic is typically between 10 and 20%, and affects the density of the image to be formed. Since the transmittance of the optical lens decreases as the image height increases, it is necessary to increase the light-emission intensity of the semiconductor laser LD in accordance with the image height, so as to supply the light-emission intensity from semiconductor laser LD where the image height is ±0 to the photoreceptor 4. Controlling the light-emission intensity such that the error generated by the shading characteristics are absorbed is called “Shading correction”.
The shading characteristics are determined by characteristics and arrangement of optical elements. At the same time, fluctuation caused in the image forming apparatus and the fluctuation caused by temperature, humidity, and ambient temperature are slight. That is, when the characteristics and the arrangement in the device are decided, a correction condition for the shading correction can be set in common in the device for all image forming apparatuses because each image forming apparatus is not required to be set separately.
The light level of the light beam to be emitted by the light source is controlled based on the level of the light beam (exposure amount) that reaches the surface of the photoconductor, for example, as described in JP-H06-255172-A. This approach, however, requires a light level detection sensor capable of detecting the light level of the light beam at the surface of the photoconductor, and an additional control circuit controls the light level of the light beam based on the detection result of the light level detection sensor. As a result, the control mechanism becomes complicated, and manufacturing cost increases.
JP-2005-11943-A proposes a semiconductor laser driver that can perform shading correction by changing a reference voltage Vref without executing APC. More specifically, an emission current In is generated by the output current of a digital analog converter (DAC). The output current of the DAC is proportional to the reference voltage Vref, and the emission current In is proportional to the reference voltage Vref. That is, the emission amount (Po) of the semiconductor laser LD is proportional to the reference voltage Vref in this example semiconductor laser driver. Without performing PAC, by adjusting the reference voltage Vref, the shading correction can be performed.
JP-2003-71510-A proposes a control method to control the quantity of the light of the light source based on correction data of the light quantity that is given in advance, corresponding to scanning position of the light spot. In this example, as a transmitting method to transmit the correction data of the light quantity to a LD modulation circuit, an output signal of the DAC is smoothed and the smoothed signal is input to the LD modulator as a light-quantity correction signal LDLVL.
Similarly, as described in the JP-2005-262509-A, a pulse width modulation (PWM) signal is smoothed by LPF, and is output to the LD controller as a light-quantity control signal.
In these examples, by inputting the light-quantity correction data and the light-quantity adjustment signal to the above-described semiconductor laser driver, the quantity of the light of the laser diode LD is determined by an analog light intensity signal that is supplied to the LD controller, and the operation of the shading correction is executed by adjusting the level of this analog signal in accordance with the position in the main scanning direction.
The position in the main-scanning direction and the quantity of the light of the laser diode LD is determined by setting a position standard in the main scanning direction based on a synchronized signal and dividing the line cycle from this position into respective areas. The analog signal is then controlled for each of the respective areas.
In the image forming apparatus 200X shown in FIG. 1, the optical unit, such as, the semiconductor laser driver 7, the laser unit 1, the polygon mirror 2, and the scanning lenses 3 are configured as a laser scan unit (hereinafter “LSU”). In order to activate the semiconductor laser LD rapidly, the semiconductor laser driver 7 and the laser unit 1 are provided in a same LD board, and disposed close to each other. It is preferable that the LD board is small so that the LD board is fit into the gaps among the LSU. By contrast, the image control unit 6 is mounted in a main board including a central processing unit (CPU), random access memory (RAM), read only memory (ROM), and an image memory.
FIG. 2 is a conceptual diagram of a related art laser diode (LD) board 7000 mounting a related art laser driver 7 and a main board 60X mounting the image control unit 6, provided in an image forming apparatus 200X. Herein, the semiconductor laser driver 7 mounted in the LD board 7000 and the image control unit 6 mounted in main board 60X are connected via a cable that is usually longer than 1 m. A supply voltage from a power supply and a ground voltage (GND) are supplied from the main board 60X to the LD board 7000 via the cable. Because a consumption current of the semiconductor laser driver 7 and a driving current for emitting the semiconductor laser LD are transmitted through voltage-transmission lines connecting to the power supply or the ground voltage, voltage down (drops) and voltage up (boosts) are generated in the voltage transmission lines by resistance of the cable.
Thus, voltage generated in the image control unit 6 differs from the voltage received in the semiconductor laser driver 7 in direct current (DC). In addition, the current fluctuates due to switching the semiconductor laser LD on and off during image formation, and accordingly the supply voltage and the ground voltage in the LD board 7000 fluctuate from point of an alternative current (AC). Thus, since the voltage fluctuates in the cable therebetween from the point of DC and AC, error may be generated in the emission amount with respect to a setting value of the quantity of the light.
In addition, recently, semiconductor lasers LD have come to be required to control the emission amount of the semiconductor laser LD with a high degree of accuracy while executing shading correction, so as to reduce the cost and increase image quality.